The present invention concerns a switched mode power supply with a timed switching regulator, whereby an electronic switching component is periodically switched on and off in such a particular pulse duty ratio/duty cycle, that an output control voltage is produced from a rectified input voltage across a storage circuit with a smoothing choke, an intermediate circuit memory backup capacitor, and a free-wheeling diode.
Such switched mode power supplies (switching regulators) are known in the art. Their advantages are: relatively high efficiency, relatively small filtering overhead because of the high clock frequency, and a large range of input voltages. The disadvantages: the use of special, sometimes very expensive, components, (among other things, special smoothing chokes with special ferrite core and possibly at least two windings), required for the control circuit. Because of the given disadvantages, the economic considerations above all, such switched mode power supplies are not yet used for all applications in which they would be well suited in principle from a technical point of view.
The present invention therefore takes as its basis, the objective of creating a switched mode power supply of the type cited, which is suitable for a larger range of applications due to a substantial reduction of the costs, yet has at least a satisfactory efficiency and reasonably high output power.
This is achieved in accordance with the invention in that, the switching component is composed of a transistor, namely a FET (field effect transistor) or an IGBT (insulated gate bipolar transistor), whose gate or base is preconnected to a special control circuit in such a way that an increased trigger current briefly flows in the trigger circuit for gating the switching component, and a substantially smaller holding current subsequently flows in the gated state, respectively. For this purpose, an active dropping resistor is appropriately preconnected to the gate, whereby, dynamically at the time when the switching component is triggered, the active dropping resistor is at least partially bridged (and thereby made lower-impedance) by the control circuit in accordance with the invention in such a way, that the relatively high trigger current flows for this time, while the bridging of the active dropping resistor is subsequently raised after the gating of the switching component, so that the relatively smaller holding current flows.
The invention is based on the knowledge that the electronic switching component (FET/IGBT) needs a relatively large trigger energy ("starting charge"), because very large internal capacitances operate between the principal current path (drain/source or emitter/collector) and the base or gate, and the switching component can only become conductive (gating control) as fast as these internal capacitances can be charged. It is therefore necessary to ensure that a sufficiently large current can flow in the trigger circuit within a very short time (few microseconds). Here, the dropping resistor could be conceived in principle as being in a correspondingly low-impedance state (for example, a current flow of at least 30 mA at 230 V), whereby it would be designed for very large dissipation power (strong heating).
It can be achieved by the present invention, however, that the active dropping resistor as a whole, i.e. when accumulated, can be of relatively high-impedance, so that the current, as a holding current, is also insignificantly small in the normal case, i.e. in the switching component's gated on state. A large trigger current is then briefly produced in practice as an amplified current pulse across the control circuit in accordance with the invention for gating the switching component, however, so that it represents a "trigger pulse amplifier" so to speak. For this, a capacitor is preferably used as an energy storage device, which (at least) briefly bridges part of an active dropping resistor, preferably designed as a voltage divider, in the gate-triggering current path or resistor path.
The invention has the advantage that no "active" circuit is needed for producing an auxiliary voltage. Rather, the circuit in accordance with the invention manages mainly with passive and therefor inexpensive components. Because of the very low power, low-priced resistance classes can be used for the resistors of the active dropping resistor. A very inexpensive switching transistor (e.g. BSR 19) can be used as the (single) active component of the control circuit in accordance with the invention.
In connection with the invention, it is furthermore preferable, if the electronic switching component forms a multivibrator circuit together with a flip-flop transistor (current mode transistor), whereby this multivibrator circuit is current-controlled by a control resistor and, therefore advantageously short-circuit protected, is triggered by a control signal. A small-signal transistor (e.g. BC 847) which is likewise very inexpensive, can be used for the flip-flop transistor. Here it is preferable for the control signal to be coupled back free of voltage potential from the side of the output control voltage, particularly by an optical coupling device, into the switching regulator's control loop. This saves a second, very expensive winding on the smoothing choke, so that a very inexpensive smoothing choke with only one winding and therefore also with a very small expense for insulation can be implemented (bar choke or choking coil annular type).
Further advantageous organizational characteristics of the invention are contained in the dependent claims and in the following description.